In the past, in the press and calendar stages of a papermaking process, a paper material was typically compressed between a pair of press rolls. However, the press rolls apply pressure to the paper material only along a narrow line. Therefore, it was difficult to increase the amount of water squeezed out of the paper material in the press part. Moreover, since the paper material is flattened at the calendar part, undesirable effects are occasionally produced on the manufactured paper. To avoid these characteristic problems, there has been a trend toward the use of a shoe press apparatus, wherein a press roll cooperates with a shoe the surface of which conforms to the outer surface of the press roll. The use of the shoe press apparatus has been growing recently, and it has come into widespread use.
FIGS. 7-9 show conventional shoe press apparatuses used in a press stage of a papermaking machine, and FIG. 10 shows a conventional shoe press apparatus used in the calendar stage. Shoe press apparatuses 100a and 100b, shown in FIGS. 7 and 8 respectively, utilize a press part P comprising a press roll R and a shoe S. A wet paper web W, a pair of felts F pinching the wet paper web W, and a belt B, are provided in the press part P. With the rotation of the press roll R, the wet paper web W, the felts F, and the belt B, run through the press part P. The arrow MD (“machine direction”) shows the direction of the rotation of the press roll R.
In many cases, a belt B is manufactured by impregnating a base body comprising a woven fabric, etc., with resin, in order to impart strength to the belt. Depending on the structure of the shoe press apparatus, a relatively long belt, as in FIG. 7, or a short belt, as in FIG. 8, may be adopted.
The shoe press apparatus 10d, used in a calendar part shown in FIG. 10, has a structure corresponding to that of the press part shown in FIG. 8, but with the press roll R replaced by a calendar roll R′. A calendar belt BC and paper material W′, having a rough surface, are sandwiched by the press part P comprising the calendar roll R′ and a shoe S.′ The belt BC and the paper material W′ pass through the press part P with the rotation of the calendar roll R′. The calendar belt BC differs in exactness from a press part belt B. However, both belts have the same basic structure, consisting of a base body and a resin.
Next, a shoe press apparatus 100c of FIG. 9 does not use a press roll. Instead, its press part P comprising a pair of shoes Sa and Sb. This shoe press apparatus is disclosed in Unexamined Japanese Patent Publication No. 131075/1998. In shoe press apparatus 100c, a wet paper web (not shown), a pair of felts F, pinching the wet paper web, and a pair of belts, Ba and Bb, pass through the press part P, between the pair of shoes Sa and Sb. Belt Ba is a driven belt.
In these shoe press apparatuses, it is important to decrease the friction generated between the shoe and the belt while the belt is running. In the shoe press apparatuses 100a, 100b, and 100d, shown in FIGS. 7, 8, and 10, the belts B and BC rotate along with the rotation of the press roll R or the calendar roll R′. Moreover, in the shoe press apparatus 100c of FIG. 9, the belt Bb rotates along with the driven belt Ba. Decreasing the friction generated between the belts and the shoes will reduce the energy consumed in driving the press roll R, the calendar roll R′, or the driven belt Ba.
Therefore, it is conventional practice to supply a lubricant between a belt and a shoe to decrease the friction generated between them. Oil is usually used as a lubricant, but water or other fluid also may be used.
FIGS. 11-14 show conventional shoe press apparatuses having lubricant supply structures. FIG. 11 is a partial cross-sectional view of a shoe press apparatus 100e, disclosed in Unexamined PCT National Phase Publication No. 503561/1997. This shoe press apparatus 100e has a lubricant feeder OS, located on the upstream side of a shoe S, and supplying a lubricant between the shoe S and a belt B from the outside of the shoe S in response to a controller.
In this shoe press apparatus 100e, the curvature of the surface of the shoe S differs from that of the press roll R so that a hollow space is provided between the shoe S and the roll R, and a lubricant supplied from outside of the shoe S can accumulate between the shoe S and the belt B.
FIG. 12 is a partial cross-sectional view of a shoe press apparatus 100f, disclosed in Unexamined PCT International Publication No. WO 00/24965. A concave part 60 is provided in the upper surface of a shoe S, and a supply passage 62, for supplying a lubricant to the concave part 60, is provided in the shoe S, leading from underneath the shoe to the concave part. Therefore, this shoe press apparatus 100f supplies a lubricant between the belt B and shoe S from the inside of the shoe S. In this connection, although a shoe S comprising two members, S′ and S″, is shown in FIG. 12, the shoe may alternatively be composed of a unitary member.
Shoe press apparatus 100g, shown in FIG. 13, is another example in which a lubricant is supplied between a shoe and a belt from the inside of the shoe. This shoe press apparatus 100g is disclosed in Unexamined Japanese Patent Publication No. 41486/1990. Two concave parts, 60a and 60b, are formed on the upper surface of the shoe S, and a lubricant is supplied to these concave parts through supply passage 62a and 62b respectively, both supply passages being provided inside the shoe S. Lubricant supplied to the concave part 60a provided in the center of the shoe S of this shoe press apparatus 100g also functions to apply pressure to the belt B.
FIG. 14 shows a shoe press apparatus 100h disclosed in Unexamined Japanese Patent Publication No. 33293/1989. In this shoe press apparatus, a guide member SA, which may move in an axial direction, is provided on a shoe S, and a lubricant feeder OS is provided on the upstream of the guide member SA. In this shoe press apparatus 100h, although the guide member SA is not pressed by a press roll, it forms the introduction part of the shoe S and may therefore be considered a member forming a part of the shoe S.
As described in the above, there are several kinds of conventional shoe press apparatus, either supplying a lubricant from the outside of the shoe S as shown in FIG. 11, or supplying a lubricant from the inside of the shoe S as shown in FIGS. 12-14. All of these shoe press apparatuses are subject to several problems. In the case in which lubricant is supplied from the outside of the shoe S, it is difficult to supply the lubricant to the area between the shoe S and belt B, since the gap between the shoe and belt is narrow, and lubricant is supplied where the shoe is in contact with the belt. Therefore, although lubricant is supplied continuously as a jet so that more lubricant may be supplied between the shoe S and belt B, it is difficult to achieve adequate lubrication since the lubricant tends to drop off before reaching the space between the shoe S and the belt B, due to factors such as the shape of the end of the shoe S.
On the other hand, in the case in which lubricant is supplied through the shoe, there is a different problem. Even though a sufficient quantity of lubricant is supplied to the interface between the belt and the shoe on the downstream side of the concave part of the shoe, little, if any lubricant is supplied to the upstream side. Moreover, although the apparatus shown in FIG. 13 partially solve this problem by supplying a lubricant at two points its manufacturing cost is relatively high since the interior structure of the shoe, and the structure of related components, are complex.